[GitHub] [incubator-daffodil] mbeckerle commented on a change in pull request #394: Feature Addition: API for traversing DSOM and generating NIFI RecordSchemas from DSOM

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mbeckerle commented on a change in pull request #394:
URL: https://github.com/apache/incubator-daffodil/pull/394#discussion_r453002919



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File path: daffodil-core/src/main/scala/org/apache/daffodil/dsom/walker/AbstractDSOMWalker.scala
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@@ -0,0 +1,217 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.daffodil.dsom.walker
+
+import java.io.File
+
+import org.apache.daffodil.compiler.Compiler
+import org.apache.daffodil.dsom.{Choice, ComplexTypeBase, ElementBase, GroupRef, ModelGroup, Root, Sequence, SimpleTypeBase, Term}
+
+/**
+ * A class designed to walk the internal representation of a DFDL Schema File.
+ *
+ * There are numerous internal event handlers to be defined by the implementing class; each
+ * corresponds to "encountering" a particular element type in the DFDL Schema file.
+ *
+ * Though recursion is used here to define the walk, it is not advised to use recursion between
+ * these event handlers.  Instead, consider a stack-like structure, as the DFDL Schema structure
+ * as well as the recursive method call structure can be represented by trees.
+ * @param schemaFile an input DFDL Schema file to parse
+ * @tparam T the return type of walkDSOMSchema and onWalkEnd.  This should be a class used to
+ *           represent the structure of the Schema.  You could always make it Unit if you don't
+ *           want these methods to return anything.
+ */
+abstract class AbstractDSOMWalker[T](schemaFile: File) {
+  /**
+   * The root element of the DFDL Schema.  This will be the starting point for the traversal
+   */
+  private final val schemaSetRoot: Root = Compiler().compileFile(schemaFile).sset.root
+
+  /**
+   * Auxiliary constructor to initialize this class with a path to a schema File rather than
+   * a File object directly
+   * @param pathToSchemaFile path to the DFDL Schema file desired to be parsed
+   */
+  def this(pathToSchemaFile: String) = this(new File(pathToSchemaFile))
+
+  /**
+   * Method to be called on the beginning of the traversal.  It is recommended to add some
+   * sort of wrapper element to a stack if you're doing a typical stack-based traversal.
+   * @param root the root element of the DFDL Schema
+   */
+  protected def onWalkBegin(root: Root): Unit
+
+  /**
+   * Method to be called when the traversal concludes.  It is recommended to put any post-processing
+   * and anything to tidy up the stack or the result here.
+   * @param root the root element of the DFDL Schema
+   * @return the result of the traversal
+   */
+  protected def onWalkEnd(root: Root): T
+
+  /**
+   * Method to be called when a *sequence* element is encountered.
+   * This is not really your array type; the typical "array" you'll see in an infoset
+   * is a special kind of Element, so it will be encountered
+   * in onElementBegin
+   * @param sequenceElement the sequence element
+   */
+  protected def onSequenceBegin(sequenceElement: Sequence): Unit

Review comment:
       The abstract walker can define a set of view mixin traits that we mix into each walked object. The traits may be specific to each walked DSOM object as it has to expose different things from them perhaps.
   
   The walker API would be defined in terms of these traits. The traits would have public members that are the things we choose to formally expose (and support) on the walker API. 




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